1. Field of the Invention
This specification relates to an active Sound Navigation and Ranging (SONAR) system and an impedance matching method thereof, and particularly, to an active SONAR system having an impedance matching circuit for driving underwater acoustic transducers in broadband and with high efficiency and the impedance matching method thereof, capable of obtaining matching circuit element values by controlling the positions and the intervals of the frequencies at which reactance components of a transducer become zero.
2. Background of the Invention
A radar, laser or infrared rays using electric waves are used to detect a target on land or in the air, but Sound Navigation and Ranging (SONAR) using acoustic waves is used to detect a target under the water. Active SONAR is essential acoustic equipment for a ship, which allows for safe sailings and serves to find an underwater (submarine) object which poses a threat to the ship. An operating principle of the active SONAR is to use a piezoelectric effect. When the current in a predetermined frequency flows on piezoelectric crystal or ceramic, the crystal or ceramic vibrates to generate acoustic waves of the same frequency. On the contrary, when external acoustic waves are received by the crystal or ceramic in the active SONAR, they generate electric energy. Frequency elements of the acoustic waves may be found through analysis for the electric energy. The equipment which can transmit or receive the acoustic waves is called ‘active SONAR.’
In order to efficiently supply maximum power at a broadband from a transmitter to a transducer as a load in the active sonar system, an impedance matching function corresponding to electric interfacing between the transmitter and the transducer is essentially required. However, most of conventional impedance matching methods are subject to a limitation of maximum efficiency up to 50% due to performing a conjugate impedance matching for transmission of maximum power to the load. Especially, these methods are not so useful to be applied to the active sonar system, which exhibits very low output impedance of its transmitter and operates within a limited range of supplied power.
Hence, in order to allow the output power in high efficiency to be taken from the transmitter, an impedance matching method, which is capable of minimizing unnecessary reactive power components by attenuating only imaginary components for the impedance of the transducer as the load, may be considered.
However, an electrical characteristic of the transducer matched by such method exhibits large deviations for an input impedance and supplied power even within the matched frequency range. Also, the deviations depend on physical characteristics of the transducer, that the control by the matching method is difficult. To solve those problems, an impedance matching method using a filter network synthesis is applied to an estimated electrical equivalent model of the transducer. However, the physically fixed parameters of the transducer cause a constraint to an impedance matching bandwidth for minimizing a reactance component, required for high efficiency operation.